This invention relates to a glass substrate for an information recording medium, and an information recording medium such as a magnetic recording medium having the glass substrate.
Conventionally, a glass substrate having a high strength for a magnetic recording medium is disclosed in Unexamined Japanese Patent Publication No. S64-42025. This glass contains, by weight, 65-75% of SiO.sub.2, 4-9% of Al.sub.2 O.sub.3, 5-10% of B.sub.2 O.sub.3, 5-9.5% of Na.sub.2 O, 0-3% of K.sub.2 O, 0-0.5% of Li.sub.2 O, 3-6% of BaO and 0.5-3% of ZnO.
Further, a glass substrate having a chemically reinforced glass for an information recording medium is disclosed in Japanese Unexamined Patent Publication No. H5-32431. This glass contains, by weight, 62-75% of SiO.sub.2, 5-15% of Al.sub.2 O.sub.3, 4-10% of Li.sub.2 O, 4-12% of Na.sub.2 O, 5.5-15% of ZrO.sub.2. In this event, the weight ratio Na.sub.2 O/ZrO.sub.2 falls within the range between 0.5 and 2.0. Further, the weight ratio of Al.sub.2 O.sub.3 /ZrO.sub.3 falls within the range between 0.4 and 2.5.
In addition, a chemically reinforced glass which contains, by weight, 58-70% of SiO.sub.2, 13-22% of Al.sub.2 O.sub.3, 5-12% of Na.sub.2 O and 2-5% of ZrO.sub.2 is disclosed in Japanese Unexamined Patent Publication No. H8-48537.
Recently, a flying height of a head has been lowered, and a rotating speed of a disk has become high with a miniaturization of the disk and a high recording density. Therefore, the strength and surface accuracy of the disk has been strictly required.
Under the circumstances, the above-mentioned conventional techniques have the following problems.
First, the glass substrate disclosed in the above-referenced Japanese Unexamined Patent Publication No. S64-42025 contains no ZrO.sub.2. Therefore, the ion exchange rate becomes slow when an ion exchange process is performed. Further, it is difficult to obtain an ion exchange layer (a compressive stress layer) having enough depth. In addition, the strength for destruction is also small.
Second, the glass substrate disclosed in the above-referenced Japanese Unexamined Patent Publication No. H5-32431 is excellent in the ion exchange performance, but contains much ZrO.sub.2. Consequently, it is difficult to dissolve ZrO.sub.2 and, further, it is impossible to remove projections of undissolved substances of ZrO.sub.2 on the glass surface even when the obtained glass is polished. As a result, the projections are often left on the surface of the glass substrate.
The obtained glass substrate may be practically applicable as a recent glass substrate for the information recording medium. However, the glass substrate may not be able to meet the demand for much higher density of the information recording medium in future. Specifically, the flying height of the magnetic head has been reduced with the increase in recording density. Consequently, the conventional glass substrate is not compatible with the magnetic head having the low flying height such as a magneto-resistive head (MR head) because the conventional glass substrate has projections on the glass surface.
Further, the conventional glass substrate contains much ZrO.sub.2 as mentioned before. Consequently, it is difficult to sufficiently increase a specific modulus because the glass has a large specific gravity. In this event, when the specific modulus becomes enough large, the burring does not easily occur even when the disk rotates at a high speed.
Finally, the glass substrate disclosed in the above-referenced Japanese Unexamined Patent Publication No. H8-48537 has a high liquid phase temperature and a low viscosity at the liquid phase temperature. Consequently, the devitrification is easily performed when a dissolved glass is formed into a plate form by the use of the downdraw molding or the press molding.